1. Technical Field
This invention relates generally to soldering or brazing and more particularly to an improved solder/braze-stop composition for preventing adherence of unwanted solder/braze material on critical surfaces of a workpiece.
Solder/braze-stops are well known to those having skill in the soldering/brazing art, and are typically employed for preventing solder or braze material from splattering or flowing onto a critical surface of a workpiece. The solder/braze-stop, sometimes referred to as stopoff, is applied to those critical areas on a workpiece where splatter or flow must be prevented. Then, during soldering/brazing, the splattered or flowing material forms a ball on the stopoff and does not penetrate to the underlying workpiece. After brazing, the stopoff and any solder/braze material thereon is wiped off.
2. Background Art
The performance of commercially available stopoffs has not been completely satisfactory. For example, a commonly available stopoff is a suspension of alumina (Al.sub.2 O.sub.3) particles in isopropyl alcohol (IPA). The performance of such an Al.sub.2 O.sub.3 +IPA stopoff is not completely satisfactory for the following reasons:
1. The stopoff dries rapidly upon application. Rapid drying causes streaking and reduces imperviousness to solder/braze. Moreover, it is difficult to double coat such a rapidly drying stopoff when required. PA1 2. The stopoff does not adhere well to a workpiece. PA1 3. Frequent agitation or stirring is required during use in order to keep the Al.sub.2 O.sub.3 particles suspended in the IPA. PA1 4. The stopoff is not totally impervious to solder/braze, so that some workpieces must be rejected. PA1 5. After brazing, the stopoff forms a hard crust which is difficult to remove without damaging the underlying workpiece. PA1 6. After removal, the stopoff leaves a residue which acts as a contaminant on the workpiece surface. PA1 1. The solder/braze-stop composition of the present invention dries readily (within about 20 seconds), yet slowly enough to avoid streaking and to allow a second coat if necessary. While it has been found that one coat of the improved solder/braze stop composition is generally sufficient, it is sometimes necessary to provide a second coat, e.g., when the first coat has been improperly applied. PA1 2. The solder/braze stop composition of the present invention adheres well to an underlying workpiece. PA1 3. Frequent stirring, agitation or any other unusual handling is not required. PA1 4. A single coat of the solder/braze stop composition of the present invention provides an impervious coating. PA1 5. After brazing, the solder/braze stop composition of the present invention forms a cake-like surface which is easily stripped from the workpiece with a damp cloth, air brush or other means. PA1 6. The solder/braze stop of the present invention does not leave contaminant residue on the workpiece.
Accordingly, commercially available stopoffs have been satisfactory for those applications which require an impervious noncontaminating stopoff which is easily handled and easily wiped off.